Plasmodium parasites cause malaria morbidity and mortality in over 40% of the worlds population. Despite a complex parasite life cycle, immunization with irradiated sporozoites can elicit sterile immunity in mice, monkeys and humans, providing a gold standard for a pre-erythrocytic stage malaria vaccine. Since sporozoites cannot be produced in vitro, efforts have focused on developing subunit vaccines that elicit immune responses comparable to irradiated sporozoites. Over the past grant period, we have shown that synthetic peptides and hepatitis B virus core protein virus-like particles (VLP) containing minimal P. falciparum CS protein epitopes, were safe and immunogenic in preclinical and Phase I trials. In the current grant, the fine specificity and function of antibody and cells elicited in volunteers immunized with ICC-1132, a VLP expressing P. falciparum CS repeats and the universal T* Th epitope, will be examined (Specific Aim 1). The protective efficacy of this VLP and linear peptide vaccines will be compared and the immune mechanisms of protection defined using transgenic parasites expressing P. falciparum CS epitopes (Specific Aim 2). In an effort to optimize humoral and cellular responses, truncated T and B cell epitopes will be tested as peptide vaccines (Specific Aim 2) and heterologous prime:boost immunization strategies, using VLP and peptides, will be assayed for protective efficacy (Specific Aim 3). A major limitation in P. falciparum vaccine Phase II trials is the inability to dissect immune responses of human volunteers to define correlates of protective immunity. We will therefore construct P. berghei transgenics expressing P. falciparum CS major or minor repeats, or the universal T* epitope, to establish small animal models for analysis of protective immune mechanisms elicited by P. falciparum CS subunit vaccines (Specific Aim 4). The information gained from these studies will be applicable to development of vaccines against malaria blood stage parasites, as well as viral and bacterial pathogens that require strong humoral responses for protective immunity.